To add words that are frequently queried in Internet search engines to make your site come up in unrelated searches. The words are often hidden as text the same colour as the background.

Probably from Wired's Jargon Watch.
"I spamdexed the word 'sex' 200 times in the home page"
by Andrew July 3, 2004
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To be spamdexed means to have violated the search engine guidelines, usually Google's, and be punished for it by being removed from the search engine or losing all visibility.

or to be spamdexed on spamdexed.com, to be reported as a violator of the search engine guidelines, usually Google's, in order to show up at the top of the search engine pages.
Yo, he tried to trick google and got spamdexed!

I did 1 billion rumrr links and spamdexed my site
by Feefee October 16, 2014
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